1. Field of the Invention
The present invention relates to an ozone generating apparatus for generating a high concentration of clean ozone gas free of nitrogen oxides (NOx) by using ultrapure oxygen as a source gas.
2. Description of the Background Art
In a conventional silent discharge ozone generating apparatus operating with a discharge gap length of about 1.0 mm or greater, a discharge occurring in a discharge space is made up of a multiplicity of micro-discharge streamers, as disclosed in, for example, Japanese Patent No. 3416982 (paragraphs 19 and 20), and Japanese Patent Application Laid-Open No. 7-277707 (1995) (paragraphs 19 and 20). In such a case, the increase in the number of locations in which the micro-discharge streamers occur promotes collisions between electrons and oxygen molecules to improve ozone generation efficiency. To increase the number of locations in which the micro-discharge streamers occur, Japanese Patent No. 3416982 discloses a dielectric having a surface resistance (surface resistivity) of 109Ω to 1013Ω. To facilitate the accumulation of electrical charges in an electrode surface, Japanese Patent Application Laid-Open No. 7-277707 discloses a high-resistance film formed on a surface of a dielectric in contact with a gap portion and having a surface resistance value (surface resistivity) of not less than 1011Ω.
It has been known that, even when the discharge gap length of not greater than 1.0 mm is provided to achieve a condition which allows the generation of an extremely high concentration of ozone, high ozone generation efficiency cannot be maintained without the addition of nitrogen, carbon dioxide or the like to a source gas if the source gas is ultrapure oxygen (having a purity of not less than 99.9%). A conventional technique for solving this problem is disclosed in, for example, Japanese Patent Application Laid-Open No. 11-021110 (1999) (paragraphs 10 and 11) showing that the high ozone generation efficiency can be maintained without the addition of nitrogen or the like by containing not less than 10% by weight of titanium oxide in terms of the proportion of the amount of metal element at least in a surface layer portion of a dielectric stacked on the surface of an electrode.
In ozone application processes, particularly ozone applications to semiconductor and liquid crystal manufacturing processes, there is a need to increase the concentration of ozone with the size reduction and integration in manufacturing processes. In addition to the increase in concentration, challenges presented are to provide a high concentration of clean ozone free of metal contamination and to provide a high concentration of ozone free of nitrogen oxides (NOx) so as to prevent corrosion and the like in a manufacturing apparatus.
The increase in the concentration of ozone has been accomplished by innovative technology development such that the discharge gap length is set at not greater than 0.1 mm and such that the action of decomposition of generated ozone is suppressed in a discharge field. The generation of clean ozone free of metal contamination has been accomplished by forming a clean discharge space covered with a dielectric so as to prevent metal from being exposed to the discharge field. For a high concentration of ozone for use in clean processes as mentioned above, ultrapure oxygen (having a purity of not less than 99.9%) is employed as the source gas for use in an ozone generating apparatus.
It has, however, been confirmed that the use of the ultrapure oxygen as the source gas makes it impossible to generate a high concentration of ozone and makes it difficult to maintain the ozone generation efficiency, resulting in performance change with time, as disclosed in, for example, Japanese Patent Application Laid-Open No. 11-021110, paragraph 5. For this reason, the common practice is to introduce an additive gas such as nitrogen and the like into the ultrapure oxygen, thereby maintaining the high concentration and high efficiency. The nitrogen that accompanies the source gas, however, inevitably gives rise to the production of NOx as a side effect in addition to a high concentration of ozone in a plasma field. It is, therefore, desired to develop a high-concentration ozone generating apparatus which does not add nitrogen to the source gas, that is, which does not produce NOx.
Experiments have shown that, for the discharge gap length of not greater than 1.0 mm or for generation of a high concentration of ozone, it is impossible to accomplish high ozone generation efficiency by employing a dielectric or a high-resistance film having a surface resistivity of not less than 1011Ω as in a conventional technique or by simply increasing the number of locations in which the micro-discharge streamers occur if ultrapure oxygen is used as the source gas.
If the discharge gap length is not greater than 1.0 mm and a photocatalytic function is imparted, it is difficult for energy derived from light to compensate for the shortage of energy derived from the discharge because the ozone generating energy derived from light is extremely low relative to the ozone generating energy derived from the discharge. It is therefore impossible for this attempt to generate a high concentration of ozone and to maintain the ozone generation efficiency. Further, this attempt lacks reliability because of wide variations depending on the method and material of a photocatalytic thin film.
In view of the foregoing, there are no practical means for generating a high concentration of ozone free of nitrogen oxides and for maintaining the high ozone generation efficiency when ultrapure ozone is used as the source gas.